This invention relates generally to tri-level imaging and more particularly to a method and apparatus for maintaining or stabilizing the white discharge level of a tri-level image at a predetermined voltage level.
In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a charge retentive surface such as a photoconductive member by first uniformly charging the charge retentive surface. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, not only are the charged (i.e. unexposed) areas developed with toner but the discharged (i.e., fully exposed) images are also developed. Thus, the charge retentive surface contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-exposed (i.e. charged) areas of the photoreceptor, as in the case of conventional xerography, while the other image areas correspond to fully exposed (i.e., discharged) areas of the photoreceptor.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) remains at or near the fully charged photoreceptor potential represented by V.sub.cad or V.sub.ddp as shown in FIG. 1a. The other images are formed by discharging the photoreceptor to its residual potential, i.e. V.sub.dad of V.sub.c (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD). The background areas are formed by discharging the photoreceptor to reduce its potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 v (V.sub.bb, shown in FIG. 1b) closer to V.sub.cad than V.sub.white resulting in a V.sub.bb of about 600 v, and the DAD developer system is biased about 100 v (V.sub.cb, shown in FIG. 1b) closer to V.sub.dad than V.sub.white resulting in a V.sub.cb of about 400 v.
As developed, the composite tri-level image initially consists of both positive and negative toners. To enable conventional corona transfer, it is necessary to first convert the entire image to the same polarity. This must be done without overcharging the toner that already has the correct polarity for transfer. If the amount of charge on the toner becomes excessive, normal transfer will be impaired and the coulomb forces may cause toner disturbances in the developed image. On the other hand, if the toner whose polarity is being reversed is not charged sufficiently its transfer efficiency will be poor and the transferred image will be unsatisfactory.
It has been observed that the aforementioned white level voltage, V.sub.w shifts differentially along the length of a photoreceptor due to variation in the charge acceptance level caused by manufacturing variations. Moreover, as a photoreceptor ages, its charge acceptance level deteriorates. To compensate for decreased charge acceptance level the photoreceptor is charged to a higher voltage level which causes shifting of the white level.
Also the element utilized for charging the charge retentive surface can become contaminated leading to reduced charging and shifting of the white level. Variation in the exposure output ROS utilized in electronically forming the tri-level image causes changes in the white level.
If the white level voltage, V.sub.w of a tri-level image drifts up or down as a result of fluctuations in the photoreceptor charge acceptance level, charging or white level exposure or any combination of the foregoing, the balance between the charged area (CAD) image represented by voltage V.sub.ddp (V.sub.cad) and the discharged area (DAD) image represented by V.sub.c (V.sub.dad)) will be upset. If the white level moves toward the discharged area image, the cleaning field for the charged image area will increase and at the same time the cleaning field for the discharged image area will decrease. This results in a tendency for less background suppression in the discharged image area and attenuation of fine line image in the charged image area.